During the time an aircraft is parked on the ground, the on board air conditioning system is generally turned off, whereas the high density of passengers, the interior lighting, the large number of windows, and the heavily insulated fuselage all contribute to raising the temperature of the cabin to uncomfortable levels. Therefore, preconditioned air is conventionally supplied to the aircraft directly into the cabin ventilation system. This may be done by connecting the aircraft either to a remote air conditioning unit, or to a portable air conditioning unit, which is towed close to the aircraft.
When connecting the aircraft to a remote ground-based air conditioning unit, a long hose is used to deliver the preconditioned air to the aircraft. Such a hose may be about 300 mm in diameter and 20 to 30 m in length. The hose is connected at one end to a connector provided on the exterior of the fuselage and communicating with the cabin ventilation system. At its other end, the hose is connected to a preconditioned air outlet of an independent air conditioning unit or of the airport air conditioning system, the preconditioned air outlet being situated about the airport terminal building, e.g. about a boarding gate. Unfortunately, the large dimensions of the hose involves large pressure drops and temperature variations. Furthermore, handling and storage of the hose when not in use is difficult.
The use of portable air conditioning units, as e.g. described in U.S. Pat. No. 5,031,690, that are towed close to the aircraft eliminates the problems associated with long hoses, since a short hose is then employed. In such a portable air conditioning unit, the cooling function is generally provided by a conventional vapour cycle refrigerant system, wherein the refrigerant compressor is driven by a diesel engine. Ambient air is sucked in the air conditioning unit and caused to flow through the evaporator of the refrigerant system so as to extract heat from this air. A blower is employed to supply the cool, preconditioned air to the aircraft at the desired flow conditions. A disadvantage of such air conditioning units is their relatively complex conception, since they include a refrigerant system and a diesel engine to operate the refrigerant system. Consequently, these units are relatively heavy and need to be either skid or truck mounted. In addition, exhaust gases from the diesel engine may be sucked into the air conditioning unit, which results in unpleasant odours for the passengers. Another disadvantage of such air conditioning units is that conventional vapour cycle refrigerant systems operate with CFCs, which are known for their harmful effect on the environment.